There has been increasing interest in converting cellulosic biomass to fuels or other chemicals. There are many known biomass conversion processes, including acid hydrolysis, enzymatic hydrolysis, and gasification. One biomass conversion process gaining traction is hydrothermal treatment, which typically includes a first step of contacting a biomass with hot compressed water, with or without an acid catalyst. This step enables the extraction and hydrolysis of hemicelluloses and, in some instances where a catalyst is used, the hydrolysis of cellulose to sugars. Depending on the time and temperature of the treatment, and the catalyst loading (if used), the sugars are either partially or completely extracted. Subsequent steps may include further treatment of the remaining unconverted biomass, as well as transformation of the extracted sugars from the first step into ethanol or other useful chemicals.
In the first step of this process, hemicellulose is converted to monomeric and oligomeric sugars, such as xylose, xylo-oligosaccharides, rhamnose, arabinose, galactose, and mannose. When acid is used in the first step, or if a subsequent cellulose hydrolysis step is employed, the cellulose can be converted largely to glucose and its oligomers (gluco-oligosaccharides). The ratio of oligomers-to-monomers varies depending on the severity of the reaction (e.g., the time and temperature history, and the catalyst amount, if used). The reaction also generates by-products, such as acetic acid, furfural, hydroxylmethyl furfural (HMF), and organic acids, such as formic acid and lactic acid.
Extracting the hemicellulose in this first step may include a digester that is adapted from the pulp and paper industry. Digesters used in the pulping industry are either large vessels in which wood chips are processed at batch-scale, or vertical vessels in which there is continuous downward movement of wood chips for processing. In some facilities, horizontal digesters with internal mechanisms for moving the biomass are also used. In the vertical digesters, the solid movement can be facilitated by a hydraulic head (hydraulic digesters) or by steam pressure (vapor phase). The vertical digesters usually have two zones to accomplish the overall task. In the zone at the top portion of the vertical digester, the biomass (e.g., wood chips) and water both move downward co-currently. In the lower zone at the bottom portion of the digester, the chips move downward, and fresh “wash” water is introduced at the bottom and flows towards the top of the digester, counter-currently to movement of the chips. The chips ultimately leave through the bottom of the digester, whereas the upper zone and lower zone liquid “extracts” are removed from middle of the digester. The hydrolysis of hemicellulose (and cellulose when specific catalysts are employed) to their respective oligomers and monomers occurs within the wood chip in the upper portion of the digester. The sugars and the by-products thus formed need to diffuse out from the chip to the surrounding extract. This may be accomplished in the “wash” or the diffusion section at the bottom of the column.
In the pulp and paper industry, the digesters are designed to extract the hemicellulose and most of the lignin from the lignocellulosic biomass by the addition of a catalyst, leaving most of the cellulose behind for pulping. Typically no measures are taken to maximize the yield of sugars extracted. The yield of sugars can be defined the following fashion:
  yield  =                              mass          ⁢                                          ⁢          of          ⁢                                          ⁢          sugar          ⁢                                          ⁢          monomer                +                  mass          ⁢                                          ⁢          of          ⁢                                          ⁢          sugar          ⁢                                          ⁢          oligomer                            equivalent        ⁢                                  ⁢        mass        ⁢                                  ⁢        of        ⁢                                  ⁢        total        ⁢                                  ⁢        sugars        ⁢                                  ⁢        in        ⁢                                  ⁢        biomass              ×    100  And, as an example more specifically for xylose, the major component of hemicellulose:
      yield    xylose    =                              mass          ⁢                                          ⁢          of          ⁢                                          ⁢          xylose          ⁢                                          ⁢          monomer          ⁢                                          ⁢                      (                          C              ⁢                                                          ⁢              5                        )                          +                                  ⁢                                  ⁢                  mass          ⁢                                          ⁢          of          ⁢                                          ⁢          xylose          ⁢                                          ⁢          oligomers          ⁢                                          ⁢                      (                          C              ⁢                                                          ⁢              5                        )                                                                ⁢                  mass          ⁢                                          ⁢          of          ⁢                                          ⁢          total          ⁢                                          ⁢          xylan          ⁢                                          ⁢          in          ⁢                                          ⁢          biomass          ×          1.13                ⁢                                        ×    100  
For processes intended for sugar production, specifically for sugars generated from hemicellulose (such as xylan), this yield needs to be maximized, especially for economic reasons. It may be possible to design a continuous digester to maximize this sugar yield, but there are attendant trade-offs, including, but not limited to, capital and engineering expenses, complicated process initiation/termination, operation complications, time to process stabilization, and unsuitability for varying amounts, types, and forms of biomass.
To overcome cost issues, smaller pulp and paper operations have used batch digesters. As mentioned above, the main goal for these operations is to delignify the biomass and make pulp, and not maximize sugars or hemicellulose extraction. A batch reactor may be used to extract lignin and hemicellulose for pulping purposes. Unfortunately, regardless of the severity of the reaction, batch reactors inherently do not maximize sugars yield, especially in situations where a catalyst is not used. For wood chips and for most forms of biomass, there will always be residual sugars trapped in the biomass, and the sugars can degrade throughout the reaction. This deficiency could be partially overcome by pressing the sugars out of biomass particles. However, a significant amount of sugars is still left behind in the biomass.
Thus, there is an ongoing need for methods for maximizing sugar yields from biomass hydrolysis. The methods of the present invention are directed toward these, as well as other, important ends.